3 Answers
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This is an interjection that the poet uses to show us that he's very happy with what is to follow.

a blessing beyond all fate.

Here the poet calls his beloved a blessing, and not just an ordinary one — he feels so blessed by the love of his beloved, that it's like he's been granted extra-special favor by God, as if he'd escaped mortality; that is the "beyond all fate." The allusion here comes by Greek mythology, and the Fates, the goddesses who were supposed to spin the story of mortals' lives on their looms; in the arms of his beloved, the author feels as if he's escaped their control.

My sole mate 'tis my soul mate.

The oomph of this line comes from the poet's managing to pun on the homophones sole and soul, and at the same time saying something profound and sweet. Sole means "only, singly", while soul is defined as the universal "essence" of a person; these are both combined with mate, so that sole mate means here "my only mate (or girl)" and soul mate means "the mate with which I share an essential emotional bond." It usually has the connotation in English of "the boy or girl I was destined to fall in love with and marry."

So, if we recapitulate everything, in my crudely paraphrased form, it is like this:

Ah, I'm happy! In this love, I've been granted an extra special blessing by God. [Why am I so happy?] Because my only mate, the only girl I'm pursuing right now, is one I share the deepest possible emotional bond with; she's the one I was "destined" for.